Transfusion of a patient with donated blood has a number of disadvantages. Firstly, there may be a shortage of a patient's blood type. Secondly, there is a danger that the donated blood may be contaminated with infectious agents such as hepatitis viruses and HIV. Thirdly, donated blood has a limited shelf life. In addition, there are some situations where blood may not be readily available, such as in a battlefield or civil emergencies.
An alternative to transfusion involves the use of a blood substitute. A blood substitute is an oxygen carrying solution that also provides the oncotic pressure necessary to maintain blood volume. Two types of substitutes have recently been studied, fluorocarbon emulsions and haemoglobin solutions.
Haemoglobin as it exists within the red blood cell is composed of two alpha globin chains and two beta globin chains, each carrying a haem molecule. One alpha-like globin chain and one beta-like globin chain combine to form a dimer which is very stable. Alpha-like and beta-like globin genes belong to a family of related globin genes which are expressed at different stages of development and regulated by oxygen tension, pH, and the development from embryo to fetus to newborn. Two dimers then line up in antiparallel fashion to form tetramers. The binding of dimers to form the tetramers is not as strong as in the case of monomers binding to associate into dimers. The tetramers, therefore, have a tendency to fall apart to form dimers and there is always an equilibrium between tetramers, dimers, and monomers. At high concentrations of globin, the predominant form is the tetramer; with dilution, the dimer becomes the predominant form. This equilibrium is also affected by solvent, salts, pH and other factors as the binding forces are partly electrostatic.
There are obstacles however to using native haemoglobin as a blood substitute. Firstly, large dosages are required, requiring large scale production of protein, either by recombinant means or from donated human or recovered non-human blood. Secondly, it is important to obtain haemoglobin that is free from infectious agents and toxic substances. Thirdly, although haemoglobin is normally a tetramer of 68,000 molecular weight, it can dissociate to form alpha-beta dimers. The dimers are rapidly cleared by the kidneys and the residence time is much too short for cell-free haemoglobin to be useful as a blood substitute.
Several approaches have been taken to circumvent these difficulties. These include the expression of haemoglobin via recombinant DNA systems, chemical modification of haemoglobin, and the production of haemoglobin variants. Haemoglobin and variants of it have been expressed in various cellular systems, including E. coli, yeast and mammalian cells such as CHO cells.
A number of naturally-occurring variants of haemoglobin are known. Variants include: variants which autopolymerize, variants which prevent the dissociation of the tetramer, and variants that are stable in alkali. There are also over 30 naturally occurring haemoglobin variants which exhibit lowered oxygen affinity. Several examples of such variants are disclosed in WO 88/091799.
Another approach to improving the use of haemoglobin is the modification of this protein by the addition of further polymers to improve the stability of the protein in the blood. For example, U.S. Pat. No. 5,900,402 describes the use of non antigenic polymers, preferably polyalkylene oxide or polyethylene glycol.
Because haemoglobins (and indeed myoglobins or other oxygen-carrying proteins) are involved in oxygen transport and storage they are, as a consequence of this function (because of the redox properties of the iron ion present in the porphyrin ring of protein), responsible for the generation of reactive oxygen species. Autoxidation of the oxy derivative (Fe(II)) leads to non-functional ferric haem (Fe(III)) and superoxide ion (O2•−), which subsequently dismutates to generate H2O2. These species can ultimately damage the protein and/or the haem group. An essential intermediate in the pathway leading to this damage is the ferryl haem (Fe(IV)=O2−), itself formed through the reaction of the haem with H2O2 and lipid peroxides. A protein/porphyrin-based radical cation (P+•) accompanies the formation of the ferryl haem from ferric haem and peroxide as set out in equation (1):P—Fe(III)+H2O2→P•+Fe(IV)=O2−+H2O  (1)
Ferryl haem and the radical can also be extremely toxic, notwithstanding their transient existence. These oxidative cascades can be damaging because: (i) peroxide is a powerful oxidant known to produce cellular damage, (ii) both the ferryl haem and protein-based radicals can initiate oxidation of lipids, nucleic and amino acids by abstraction of hydrogen atoms, and (iii) haem modification can lead to highly toxic haem to protein-cross-linked species and to the loss of haem and the release of the ‘free’ iron.
The potential for haemoglobin-mediated peroxidative damage exists especially whenever the protein is removed from the protective environment of the erythrocyte. This would occur, for example, during spontaneous erythrocyte haemolysis or in haemolytic anaemias (e.g. sickle-cell anaemia). It has been shown that myoglobin induces kidney damage following crush injury (rhabdomyolysis) by exactly this peroxidative mechanism, rather than by free-iron catalysed Fenton chemistry as was thought previously (Holt et al, (1999) Increased lipid peroxidation in patients with rhabdomyolysis. Lancet 353, 1241; Moore, et al (1998) A causative role for redox) cycling of myoglobin and its inhibition by alkalinization in the pathogenesis and treatment of rhabdomyolysis-induced renal failure. J. Biol. Chem. 273, 31731-31737).
It has also been shown recently that haemoglobin can cause similar damage in vivo when it is released from the erythrocyte in subarachnoid haemorrhage (Reeder, et al (2002) Toxicity of myoglobin and haemoglobin: oxidative stress in patients with rhabdomyolysis and subarachnoid haemorrhage. Biochem. Soc. Trans. 30, 745-748). Furthermore, uncontrolled haem-mediated oxidative reactions of cell-free haemoglobin (developed as a blood substitute) have emerged as an important potential pathway of toxicity, either directly or via interactions with cell signalling pathways (Alayash, A. I. (2004) Oxygen therapeutics: can we tame haemoglobin? Nat. Rev. Drug Discovery 3, 152-159). The toxicity of ferryl haemoglobin has been demonstrated in an endothelial cell culture model system of ischaemia/reperfusion [McLeod, L. L. and Alayash, A. I. (1999) Detection of a ferryl-haemoglobin intermediate in an endothelial cell model after hypoxia-reoxygenation. Am. J. Physiol. 277, H92H99] and in cells that lack their antioxidant mechanisms such as glutathione (D'Agnillo & Alayash (2000) Interactions of haemoglobin with hydrogen peroxide alters thiol levels and course of endothelial cell death. Am. J. Physiol. Heart Circ. Physiol. 279, H1880-H1889).
Ferryl haemoglobin can cause cell injury, including apoptotic and necrotic cell death. Perfusion of rat intestine with chemically modified haemoglobin has been shown to cause localized oxidative stress, leading to leakage of the mesentery of radiolabelled albumin (Baldwin et al (2002) Comparison of effects of two haemoglobin-based O2 carriers on intestinal integrity and co microvascular leakage. Am. J. Physiol. Heart Circ. Physiol. 283, H1292-H1301). Importantly, the cyanomet derivative of this haemoglobin, in which the haem iron is blocked with cyanide and is unavailable to enter a redox reaction, produced no cellular changes.
U.S. Pat. No. 5,606,025 describes the conjugation of haemoglobin to superoxide dismutase and/or catalase as one approach to reduce reperfusion injuries and other free-radical mediated processes associated with haemoglobin blood substitutes.